Code wise you are required to use a fan-rated electrical box. Use a fan made for the damp/wet area. Could mean additional wiring to have separate switches for fan and light. Never use a lighting dimmer as a speed control for a fan. Use a fan speed control. When in doubt, hire a pro. Unless your buddy or brother-in-law really IS an electrician, don't let him wire anything.
Depending on what you are mounting the fan to, it is very easy, or quite difficult. The problem lies if there isnt already proper wiring and support for a fan, then you have install a brace and/or wiring, etc. If there is already box there that can safely hold a fan, go for it!